1. Field of the Invention
The present invention relates to loudspeakers, and in particular to flat-panel loudspeakers or flat-panel sound transducers.
2. Description of Prior Art
The tendency which is evident in home entertainment products towards ever smaller and ever more compact components also applies to loudspeaker technology. The trend even goes as far as suggesting that loudspeakers should not only be small, but also “invisible” to the listener, i.e. hidden from the listener's eyes. The possibility of invisible installation is very useful particularly for multi-channel playback, such as surround, and for wave-field synthesis (WFS). The number of individual channels and thus loudspeakers required herefore rapidly amounts to more than 50 items. However, since such playback systems are also to be developed and offered for home use, and since it must be assumed that the customer, for space reasons, does not wish to fit 50 conventional loudspeakers into his/her living room for, e.g., a WFS system, alternative loudspeakers will have to be employed.
The aim is to design loudspeakers such that they may be integrated with other pieces of equipment or furniture, so that in this manner, they may be distributed across the rooms in an inconspicuous manner. For example, there have already been loudspeakers that act as picture frames, as monitors or even as doors of wardrobes at the same time.
Cone loudspeakers are not suitable for technical implementation of these “hidden” loudspeakers, since cone loudspeakers are not flat enough due to their diaphragm shape. A loudspeaker whose diaphragm is flat as a plate to start with and whose electroacoustic excitation system is as small as possible in terms of dimensions is more suitable. This principle, i.e. the use of a plate as a diaphragm in connection with the use of an excitation system, has already been employed in DE 465189, published in 1929, and its supplements DE 484409 and 484872 for acoustic shop-window advertising. Then, a window pane of a shop window served as a diaphragm which was excited by means of an attached electrodynamic excitation system so as to reproduce sound.
The functional mechanism underlying this principle is that an electrical signal applied to the electrodynamic excitation system is transformed to a mechanical audio-frequency vibration. At an excitation point, where the excitation system is present at or fixed to the diaphragm, this mechanical vibration is transferred to the plate serving as the diaphragm, whereby structure-borne sound is produced in the plate. It is in particular that portion of structure-borne sound which propagates in the diaphragm by means of bending waves that provides for the generation of air-borne sound.
With this loudspeaker principle, the generation of air-borne sound consequently is effected via the indirect way of structure-borne sound. Unlike with cone loudspeakers, the longitudinal mechanical vibrational motions of the vibrational pulses of the excitation system are not taken over by the diaphragm and immediately translated into air-borne sound, but structure-borne sound is initially created in the diaphragm, which—in particular, the ending-wave portion of same—subsequently excites the surrounding air to form longitudinal waves, or compressional waves, i.e. sound. The transformation of structure-borne sound to air-borne sound here acts like a filter in the chain of signals. As a result, only that portion of the signal to be reproduced which may propagate as structure-borne sound in the plate and may subsequently be radiated off into space is reproduced as air-borne sound.
Since, as has already been mentioned, that portion of structure-borne sound that propagates in the form of the bending wave makes the largest contribution to generating air-borne sound by means of a plate diaphragm, the properties of the bending wave, in particular its excitation and propagation, have a decisive impact on the design of a flat-panel loudspeaker in accordance with the bending-wave principle. If these properties are taken into consideration, this results in the fact that for broad-band sound reproduction, low-weight and large-size diaphragm plates are required. The plate size required, however, conflicts with the aim of invisible integration of the loudspeaker into the surroundings of the listener. As an example, the reproduction of the frequency range below about 200 Hz is of poor quality with relatively large plates. The reason for this is that a plate resonates only in its eigenmodes with its associated natural frequencies, and that the mode densities, i.e. the number of modes per frequency range, is decisive for sound reproduction. However, sufficient mode density has not been achieved so far below 200 Hz.
Thus, there is a need for a loudspeaker which is amenable, on the one hand, to invisible integration, i.e. which may be implemented to be flat and small, and which, on the other hand, enables satisfactory sound reproduction not only in the medium- and high-tone ranges, but also in the low-tone, or bass, range.
DE 19541197 A1 describes a cone loudspeaker having an electrodynamic vibration system, a cone-shaped diaphragm, a surround and a basket where the diaphragm is suspended above the surround. When a sound signal is applied to the vibration system, the diaphragm performs an upward movement along the center line. The diaphragm is provided with a layer of a piezoelectrical material which is also connected to the sound-signal source and experiences changes of extension in the process. Depending on whether the layer is connected to a further layer or is a bimorphous arrangement of two longitudinally and/or radially vibrating plates which are oppositely poled and glued to one another, the layer acts as a thickness vibrator or as a bending vibrator.
DE 19960082 A1 describes a loudspeaker having a plate diaphragm driven by a vibration drive at its back. During the vibration the plate diaphragm performs an upward movement.